Over the last fifteen to twenty years or so, there has been substantial interest in automotive-type, lead-acid batteries which require, once in service, little or no further maintenance throughout the expected life of the battery. This type of battery, often termed a "maintenance free battery", was first commercially introduced in 1972 and is currently in widespread use.
There has been a considerable amount of attention addressed to the type of alloys used in maintenance-free batteries. When the maintenance-free batteries were first commercially introduced, conventional automotive lead-acid batteries commonly used grids formed from antimony-lead alloys in which the antimony content ranged from about 3-4.5% by weight of the alloy composition. Such alloy compositions were capable of being formed into battery grids by gravity casting techniques widely used in the 1970's. Moreover, the batteries made using grids of those alloy compositions had desirable deep discharge cycling characteristics.
Unfortunately, such high antimony content lead alloys could not be used for grids for maintenance-free batteries. The use of such alloys resulted in batteries having undesirable gassing characteristics. In other words, grids made from such alloys accepted an excessive current during constant voltage overcharge so that excessive gas generation occurred. Accompanying this gas generation was the loss of water from the sulfuric acid electrolyte. Much commercial interest for alloys for maintenance-free batteries centered around calcium-tin-lead alloys and "low antimony" lead alloys--i.e., the antimony contents in such alloys being in a range of about 1-2% by weight or so.
In conventional lead-acid battery construction, a strap is cast onto the lugs located on the battery plates to electrically connect the plates of the same polarity together. This cast strap typically includes a portion, often termed a "tombstone" because of its shape, which is positioned adjacent to an aperture in the battery container cell partition. Adjacent tombstones and associated straps which connect plates of opposite polarity are initially assembled on either side of the aperture and are then welded to form an intercell weld in a through-the-cell partition configuration. This intercell weld then serves as the current path from one cell of the battery to the adjacent cell. As used herein, the term "strap" or "battery strap" refers to the strap connecting the lugs of the respective battery plates as well as the portion used to form the intercell connection.
Whether the lead-acid batteries were of a maintenance-free type or not, the intercell connection has been of substantial concern to battery manufacturers. Thus, a considerable amount of technology has developed over the years in an effort to provide a reliable, through-the-cell intercell connection.
One type of technology has been termed an "extrusion-fusion" welding process. In this process, the tombstone is first extruded under cold flow conditions into the aperture in the cell partition. The extruded portions are then fused using electrical resistance heating. Many other techniques are known for forming the intercell connections, among these being processes in which the intercell weld is created principally or solely by fusion.
Crucial to any of the processes by which the intercell connection is made is the need to have an electrolyte-tight seal between the portion of the strap forming the tombstone and the partition wall. Such a tight seal is needed for many reasons. It is thus desired to prevent any path for electrolyte from one cell to another that would create, in effect, a minor short-circuit path. Without cell-to-cell electrolyte isolation which would be compromised by even a minor short circuit path, the desired and correct maintenance of the battery voltage is likewise compromised. Additionally, and importantly, when intercell welds corrode and fail, the potentiality for explosions exists as is well known.
Maintaining the electrolyte-tight seal throughout a satisfactory battery service life is quite difficult. Thus, the intercell weld is typically submerged to some extent in the electrolyte. Accordingly, intercell corrosion problems can become a significant concern.
It is, of course, well recognized that lead-acid batteries are perishable products. Eventually, such batteries will fail; and there are several possible failure modes, e.g.--due to positive grid corrosion. The thrust of maintenance-free batteries has been to forestall the failure in performance for a period of time commensurate with the expected life of the battery, e.g.--three to five years or so. However, for the reasons evident from the foregoing, it is highly desirable, if not perhaps essential, to have the eventual failure mode be other than failure due to faulty intercell connections.
In the past few years, there have been several factors which have complicated the situation. One is seemingly ever-increasing power and energy requirements for SLI automotive batteries. Many factors have contributed to the need and/or desire for batteries having more power.
Yet another complicating factor is the "under-the-hood" space requirements. Automobile manufacturers have lessened the space available for the batteries. Typically, it has become necessary to provide lower profile batteries--i.e., batteries having a less overall height than previously used.
These complicating factors of increasing power and less available space have required battery manufacturers to alter the internal configuration and designs to provide the power and energy needed in the desired low profile container. This has typically involved increasing the number of plates per cell and decreasing the thickness of the battery grids. For example, the number of plates in a BCI Group 24 battery over the past few years has increased from about 13 to about 19 or so, while the thickness of the positive grids has decreased from about 70-75 mils down to 55 mils, and even 45 mils or so. This has allowed battery manufacturers to provide batteries having relatively high rated capacities.
What has also occurred in the recent years for various reasons is a substantial increase in the vehicle under-the-hood temperature to which an automotive SLI battery is exposed. This increased temperature obviously presents a particularly acute situation in the warmer climates. One battery manufacturer has perceived that, in the past three years or so, the temperature in such warmer climates to which an SLI battery in service is exposed has risen from about 125.degree. F. to about 165.degree. F. in new automobiles.
The specific temperature increase to which SLI batteries are now exposed is not per se of particular importance. What is important is that the under-the-hood temperatures have in fact increased. The impact of this rise in vehicle under-the-hood temperatures on the failure modes and the timing of such failures has been substantial. The incidence of premature battery failure due to failure of intercell welds has been significant. The industry has failed to appreciate the impact of all of these complicating factors on current maintenance-free battery designs and their performance and useful service life.
One attempt to deal with the acute problem of the high under-the-hood temperatures has been to retrench. Thus, one automotive battery manufacturer has developed a battery specifically directed for use in high heat environments in which thicker positive grids are used, less plates per cell are used and the head space in each cell is filled with hollow plastic microspheres. The presence of such microspheres may perhaps be perceived to function as a vapor barrier to electrolyte to minimize evaporative loss of water in the electrolyte or for limiting heat transfer or for perhaps some other purpose.
A wide variety of strap alloys have been used over the years in maintenance-free and in other SLI battery applications. More typically, these lead-based alloys include antimony, arsenic and tin in a wide variety of levels together with other alloying ingredients such as copper, sulfur and selenium. Typically, the antimony content has ranged from about 2.7 to about 3.4% by weight of the total alloy. One prior alloy of this general antimony content also included, arsenic in the range of 0.13-0.2%, tin in the range of 0.3-0.4% and selenium in the range of 0.013-0.02%. Another antimony-lead alloy of this type also included arsenic in the range of 0.16-0.19%, tin in the range of 0.14-0.16% with copper in the range of 0.05-0.06% and sulfur in the range of 0.0007-0.0017%. Still another antimony-lead alloy used in an SLI automotive battery included arsenic at a level of 0.07%, tin at 0.06% and copper at 0.037%. Lastly, still another strap alloy of this type used in an SLI automotive battery included arsenic at a level of 0.005%, tin at a content of 0.005%, selenium at 0.008%, copper at 0.003% and sulfur at 0.0006%.
In view of the complicated situation during service which has caused a significant increase in premature battery failures due to faulty intercell welds, there is a substantial need for a solution that will provide automotive SLI batteries for recent model automobiles which are capable of performing even in the warmer climates with satisfactory service lives.
It is accordingly an object of the present invention to provide a maintenance-free, lead-acid battery capable of satisfactory service life even when exposed to the relatively high temperature under-the-hood environment in recent model automobiles.
Another and more specific object lies in the provision of an alloy composition that may be used for making the straps for such maintenance-free batteries.
A still further object provides a strap alloy for such batteries that imparts to the batteries enhanced resistance to corrosion in comparison to alloys presently being used.
Yet, another object of the present invention is to provide an SLI automotive lead-acid battery in which the eventual principal battery failure mode is a mode other than faulty intercell welds.
Other objects and advantages of the present invention will be apparent as the following description proceeds, taken in conjunction with the accompanying drawings.